This event is generated by the Microsoft Hyper-V application.
It is not returned by a NetVault process but by the VDS Basic Provider.
As per this Microsoft KB:
"The Virtual Disk Service (VDS) looks for the Location paths property of the controller. The value of this property is the path location of the disk. However, a virtual controller does not have this property."
"This event can be safely ignored."
Also NetVault Backup Plug-in for Hyper-V will require volume snapshots to be produced and mounted before backup and as per this Microsoft KB:
"When a new volume snapshot is created, the Virtual Disk Service calls the IOCTL_STORAGE_GET_DEVICE_NUMBER storage I/O control code to obtain the device number of the new volume. However, because there is no device number associated with a volume snapshot, the Virtual Disk Service provider generates an error and then logs an Error event in the System log."